The concept of underwater hydrocarbon processing facilities has been developed by different operators with the aim of streamlining the production of hydrocarbon from underwater wells. In general, the underwater hydrocarbon processing facility is part of a plant including topside hydrocarbon processing facilities, and long distance transport pipelines. The exploitation of underwater oil and/or gas hydrocarbon reservoirs, through underwater hydrocarbon processing facilities, foreseeing the collection and transport of the hydrocarbon up to topside facilities or to shore, is known. The development of underwater hydrocarbon processing facilities has rapidly increased in recent years and it is likely to boom in the near future. These underwater hydrocarbon processing facilities can be located from shallow to deepwater and in any geographical areas of the world in mild or harsh environment. The topside facilities can be located on offshore fixed or floating vessels or onshore and that can be relatively near, relatively far or relatively very far from the reservoir. The underwater field development schemes/configurations have become more complex because of the increasing number of requirements and by the need of reducing the project development costs to make the exploitation of the fields economically attractive. This issue is even more important in deepwater and/or when the topside facilities are relatively far or relatively very far from the field.
The recent developments of underwater technology processing devices and the great interest of oil companies boosted the feasibility of complex schemes. For examples, the recent development of underwater active processing devices broadens the potentiality of the underwater hydrocarbon processing facilities to cover nearly all the processes of a plant. The active underwater processing devices are configured to perform the following activities: boosting the hydrocarbon (single or multiphase), separation or water treatment or combination of these processes. Consequently the main underwater processing activities are the following: liquid boosting, multiphase boosting, underwater separations (liquid/liquid, Gas/liquid, oil/water/gas), wet or dry gas boosting, water treatment, heat exchange and injection. Prototypes of underwater processing devices were designed and built since the 1970's but the first industrial applications were performed in the 1990's starting with single phase and multiphase subsea pumps. In the 2000's the first subsea separation station was installed in the North Sea. However, underwater processing devices still have limited Mean Time Between Maintenance (MTBM) and require frequent intervention. Moreover, underwater hydrocarbon processing facilities have to be adapted to the evolution of the field because the process parameters change during the field life. Adaption means changing and/or expanding the production rate of the underwater hydrocarbon processing facilities that shall be configured to fulfill also this need. The oil companies exploiting the fields have the goals of increasing the production uptime and reducing the lifecycle costs (CAPEX+OPEX).
Additional information on the current status of underwater hydrocarbon processing facilities can be found in the OTC 24307 paper “STEPS TO SUBSEA FACTORY” by Rune Mode Ramberg (Statoil), Simon R. H. Davies (Statoil), Hege Rognoe (Statoil), Ole Oekland (Statoil).
Many advantages correlated to underwater hydrocarbon processing facilities are listed in the above-reference paper that, among others, includes:                Increase hydrocarbon recovery and accelerate production;        Greater energy efficiency because the location is closer to wells;        Increase lifetime of existing installations;        Reduce topsides space and weight when starting up new subsea fields.        
While underwater hydrocarbon processing facilities bring many great advantages, the downsides are that the construction and the maintenance of an underwater hydrocarbon processing facility are rather complex with a degree of complexity that increases with the water depth or with the peculiar characteristics of the hydrocarbon field. The underwater hydrocarbon processing facilities are currently configured and built in accordance to two types of architecture: the single block architecture, and the multiple blocks architecture. The underwater hydrocarbon processing facilities organized according to the single block architecture have the drawback of being relatively heavy and each processing device is hardly replaceable by an analogous device. The underwater hydrocarbon processing facilities organized according to the multiple blocks architecture, such as that disclosed in U.S. Pat. No. 6,640,901, call instead for a large number of connections between blocks and the interfaces between blocks and are thus rather relatively complicated.
In addition to that, both architectures do not offer an adequate flexibility for relatively easily adapting the underwater hydrocarbon processing facility to different demands. Furthermore, installation, inspection, maintenance and retrieval of blocks can be particularly relatively demanding tasks.